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Euoplocephalus
Euoplocephalus is an anklyosaur from the Cretaceous. It was one of the largest anklyosaurs. Etymology The genus name Euoplocephalus, meaning "well-armed head", is derived from the Greek words eu (εὖ) meaning "well", hoplo~ (ὁπλο~) meaning "armed", and kephale (κεφαλή) meaning "head".2 This name has been misspelled more than a dozen different ways in formal scientific literature. The specific name tutus means "safely protected" in Latin. The only valid species known today is Euoplocephalus tutus. Description Among the ankylosaurids, Euoplocephalus was exceeded in size only by Ankylosaurus, and perhaps Tarchia and Cedarpelta. Euoplocephalus was 6 metres (20 ft) long and weighed about 2 tonnes (2.2 short tons).3 It was also 2.4 metres (7.9 ft) wide. Like other ankylosaurids, it had a very broad and flat low-slung torso, about four feet high, positioned on four short legs. The skull of Euoplocephalus can be distinguished from most other ankylosaurids by several anatomical details, including: the pattern of bony sculpturing in the region in front of the eyes; the form of the palpebral bones (small bones over the eyes),4 which may have served as bony eyelids;5 the shallowness of the nasal vestibule at the entrance of the nasal cavity;4 the medial curve of the tooth rows in the upper jaw; and the teeth, which are relatively small, lacking true cingula, and having variable fluting of the denticles.4 However, these traits are shared with a number of closely related forms, some of which in the past have been included in the genus. The skull resembles a truncated equilateral triangle when viewed from above, and is slightly wider than it is long. The largest known skull, that of specimen AMNH 5403, has a length of 411 millimetres and a width of 478 millimetres. Most skull elements are completely fused and two skull openings normally present with dinosaurs, the antorbital fenestra and the upper temporal fenestra, have closed. The skull has nineteen to twenty-four teeth in each upper jaw. The frontmost snout bone, the premaxilla, is toothless. The teeth are very small, with a maximal height and width of just 7.5 millimetres. The strongly drooping snout is blunt, wide and high, and filled with very complex air passages and sinuses, the form and function of which are not yet completely understood. Each side has two external nostrils. The lower jaw has a very concave upper rim with twenty-one teeth. At its front a short low extension is present, to provide contact with the wide predentary, the bony core of the lower horny beak, that fitted within the upper beak of the snout. As in most quadrupedal ornithischians, its neck is moderately short. The scapula is massive and robust, and the very robust forelimbs are shorter than the hindlimbs.7 The tail is long and ends in a bony club. Old restorations of Euoplocephalus and rejected synonyms (Dyoplosaurus, Scolosaurus) often show a club with two large vertical spikes. This is an error based on a restoration of Scolosaurus by Franz Nopcsa; the specimen he used had an incomplete tail that stopped just beyond the pair of conical spikes now known to have been positioned halfway along its length. He restored the tail as ending just after the spines. Other artists combined the spikes with the tail club, compounding the inaccuracy.7 The narrow distal half of the tail is stiffened by bundles of ossified tendons. The vertebral column of Euoplocephalus is made up of at least seven cervical vertebrae, at least eleven "free" dorsal vertebrae, typically four sacrodorsals forming a fused "sacral rod" in front of the sacrum proper, three "true" sacral vertebrae, and between one and four caudosacrals.6 Like with other ankylosaurians,1 the last four dorsal vertebrae and the first caudal vertebra are thus fused to the sacrum, forming a reinforced synsacrum of at least eight vertebrae,7 the spines of which form a fused supraneural plate, also incorporating the zygapophyses. There are at least twenty-one caudal vertebrae; the total number of caudal vertebrae is uncertain because approximately ten are fused to form part of the tail club,7 bringing the total to about thirty. This fusion is also seen in other ankylosaurids; it is possible that the extent of fusion is an age-related feature. The humerus is very robust with strongly expanded upper and lower joints, combined with a narrow shaft. On the upper shaft an enormous deltopectoral crest is present of which the lower part does not gradually merge with the shaft but is warped to the front, forming a thick knob or lip. All this indicates a very heavy musculature. In the lower arm the robust ulna has a well-developed olecranon process. The wrist and hand bones are not well known. In the pelvis, the front blade of the ilium splayed out to the front, reaching all the way to the widest point of the belly to support the gut. This blade also forms a bone shelf at the rear side of the body. The rear blade of the ilium is shorter than the diameter of the hip socket it was located behind, meaning the leg is located at the rear end of the pelvis, near the tail base and much closer to the midline than the belly sides. The pubic bone is unknown. The ischium is a short, curved, vertically positioned bone strap. The thighbone is short, robust and straight with a low fourth trochanter positioned below the midpoint of the shaft. The robust shinbone is shorter than the thighbone. The foot is not well known but functionally tridactyl with hoof-shaped instead of sharp claws.6 Armor The head and body of Euoplocephalus were covered with bony armor, except for parts of the limbs and possibly the distal tail. The armor consisted of osteoderms, skin ossifications that are not part of the skeleton proper. This armor was in 1982 extensively described by Kenneth Carpenter, who however, largely based himself on the very complete specimen NHMUK R5161, the holotype of Scolosaurus,7 which genus no longer is seen as a synonym of Euoplocephalus. When limited to the certain material of the latter genus, little is known of the exact configuration of the armor, with the exception of the head and the neck. The most informative specimen in this respect would then be ROM 1930, having conserved some osteoderms of the torso in their original position.6 In any case, much of the armor was made up of small ossicles,10 bony round scutes with a diameter of less than five millimetres, of which often hundreds have been found with a single specimen. If the armor was configured in an identical way to that of Scolosaurus, many of these small ossicles had fused into a kind of pavement, forming transverse bands on the body. The banded arrangement is thought to have permitted some freedom of movement.7 Four of these bands might have been present on the anterior half of the tail, three on the pelvis, perhaps fused into a single "sacral shield", and four across the front part of the torso. Inset in these bands were horizontal rows of larger oval, flat or keeled, scutes. Types of large scutes varied by body region. It might be that the scutes on the shoulder, near the midline of the body, were largest and tallest; ROM 1930 includes some osteoderms with a base length of fifteen centimeters. Little is known about the armor of the limbs. Large keeled plates were present on the upper arms as shown by specimen TMP 1997.132.01 conserving a round osteoderm near the humerus with a diameter of twenty centimeters and narrower spikes associated with the lower arm. The neck was protected by two bone rings, open at the underside, that are called "cervical half-rings". Earlier seen as a fusion of osteoderms,7 this was doubted by Arbour et al. in 2013, who pointed out that they formed a lower layer, possibly consisting of ossified cartilage, as indicated by a smooth surface and a woven bone texture. Each half-ring is constructed out of six rectangular concave plates, three per side. Each plate has a large keeled osteoderm on top, often not fused with it. With Euoplocephalus, these neck osteoderms do not have smaller osteoderms at their bases, and their keels do not overhang their posterior edges.6 The armor of the skull consists of a large number of ossicles, called caputegulae ("head tiles"), that have fused with the normal skull elements, largely fading their sutures. On the snout they form a chaotic and asymmetric mosaic. On the rear nasal region, at the midline a single hexagonal larger plate is present. A keeled plate per side, somewhat more to the back forms the snout rim. Behind the level of the eye sockets the caputegulae fuse into a single bone surface. The upper rim of the eye sockets is formed by two pyramid-shaped osteoderms pointing to the sides and rear. In addition, Euoplocephalus had two pyramid-shaped squamosal "horns" growing from the back corners of its head. Between them the nuchal crest is covered by two osteoderms per side. At the lower rear side of the skull, a quadratojugal horn is present, in the form of an enormous tongue-shaped osteoderm projecting below. Discovery and species Category:Ankylosaurs Category:Cretaceous dinosaurs Category:Dinosaurs of North America